Viruses, microorganisms and scientists meet the nuclear pore. Leysin, VD, Switzerland, February 26-March 1, 1998.

The modular architecture of the nuclear pore complex (NPC) consists of a membrane-embedded basic framework (the ‘spoke complex’) made up of eight multidomain spokes with two rings on each face (Figure 1A). The ring facing the cytoplasm is decorated with eight ~50 nm fibrils extending into the cytosol, and the nuclear ring is capped with a basket-like assembly of eight thin, 50–100 nm filaments joined distally by a 30–40 nm diameter terminal ring. The center of the basic framework harbors the ‘gated transport channel’, which is involved in signal-mediated bidirectional transport of macromolecules and is plugged to the central ‘transporter’, whose ultimate structure and functional role in mediated nuclear transport remains to be established. N.Panté and U.Aebi (Basel) reported their approaches to the functional roles of the different NPC components and to dissection of the steps of nuclear import and export at the NPC by direct visualization of gold-labeled proteins and RNAs moving in or out of the cell nucleus through the NPC (Figure 1B). Since nucleic acids are not transported through the nuclear pore per se, the nuclear–cytoplasmic transport of the genomes is mediated by saturable transport receptors shuttling between nucleus and cytoplasm (Görlich, 1997; Mattaj and Englmeier, 1998). These receptors recognize and bind to nuclear localization signals (NLSs) and nuclear export signals (NESs) presented by the transported molecules and mediate their translocation through the NPC. An import receptor, for example, binds to its substrate in the cytoplasm and carries it through the NPC into the nucleus. An exception here may be the plant import receptor (Merkle et al., 1996; Smith et al., 1997), which is still found connected to isolated nuclei (T.Merkle, Freiburg). On the nuclear side, the imported substrate is released and the receptor returns to the cytoplasm to initiate additional rounds of import (E.Izauralde, Geneva; Figure 2). Conversely, an export receptor binds its export substrate in the nucleus and releases it in the cytoplasm (Fornerod et al., 1997; Kutay et al., 1997, 1998).